1. Field of the Invention
This invention relates to a polyimide having a low elastic modulus and superior heat resistance and mechanical characteristics, and a process for producing the same.
2. Description of the Related Art
In general, polyimides are superior in heat resistance, electric characteristics and mechanical characteristics; hence they have been used in broad application fields for heat-resistant, insulating materials in the field of electronic equipments, films, mechanical parts and further, structural materials, etc. However, conventional highly heat-resistant polyimides i.e. totally aromatic polyimides are highly elastic and inferior in the processability. Recently it has been earnestly desired to improve the material characteristics thereof as advanced techniques including semiconductor techniques are developed, and polyimides having a low elastic modulus have been needed; thus a polyimide having high heat-resistance and a low elastic modulus has been desired.
In order to satisfy such needs, a totally aromatic polyimide having a low elasticity has been proposed in e.g. Japanese patent application laid-open Nos. Sho 63-199237/1988 and Sho 63-199238/1988. Further, a low elastic polyimide having a siloxane chain introduced thereinto has been proposed in Japanese patent application laid-open No. Sho 63-35625/1988.
The above-mentioned Japanese patent application laid-open Nos. Sho 63-199237 and Sho 63-199238 disclose reduction in the elasticity of polyamides in the case of a totally aromatic polyimide, but such a polyimide has a tensile, elastic modulus around 200 Kgf/mm.sup.2 which is, however, a still insufficient value. Further, according to Japanese patent application laid-open No. Sho 63-35625, a polyimide having a tensile elastic modulus lower than 200 Kgf/mm.sup.2 is obtained using a silicone diamine, but its values of tensile strength and elongation are insufficient. Further, both the former and latter polyimides cannot be obtained in the form of polyimide solution. Thus, in order to obtain polyimides, it is necessary to bake a solution obtained in the form of a polyimide precursor, at a high temperature and for a long time; hence this is commercially uneconomical. Japanese patent application laid-open No. Sho 61-118424 discloses a polysiloxane chain-containing and solvent-soluble polyimide, which, however, cannot be regarded as sufficient in the aspect of heat resistance.
As described above, a polyimide having an elastic modulus lower than 200 Kgf/mm.sup.2 and superior heat-resistant characteristic and mechanical characteristic both intrinsic of polyimides cannot be found in the prior art.
The present inventors have made extensive research in order to solve the above-mentioned problems and have achieved the present invention.